SEPANG, Malaysia >> As military aircraft and a flotilla of ships from a half-dozen nations combed the waters south of Vietnam on Sunday for signs of a jet with 239 people onboard that vanished a day earlier, the authorities here deflected troubling questions about two passengers on the flight who had used passports listed in an international database as lost or stolen.

The secretary-general of Interpol, Ronald K. Noble, said Sunday that no checks had been conducted by the authorities in Malaysia or any other country about the two passports before the plane, a Boeing 777-200, left on Flight MH370, which disappeared Saturday en route from Kuala Lumpur to Beijing.

"This is a situation we had hoped never to see," Noble said. "For years, Interpol has asked why should countries wait for a tragedy to put prudent security measures in place at borders and boarding gates."

On a second day of search-and-rescue operations punctuated by false and unconfirmed sightings of debris from a possible wreckage, an airline official told relatives of the passengers of Flight MH370 to "expect the worst," and electronic records emerged linking the passengers with stolen passports to the same travel agency in Thailand.

Malaysia Airlines confirmed that the missing aircraft had been involved in a collision with another plane in 2012 at the Shanghai airport that resulted in damage to the Malaysian aircraft’s wingtip. But the airline said the wing was repaired by Boeing and declared safe to fly.

Security and aviation experts continued to offer starkly disparate theories about what happened to the missing plane. In a series of briefings, Malaysian officials refused to answer any questions relating to what they described as "security matters."

In China, home to a majority of those aboard the flight, there were signs of anger and frustration over Malaysia’s handling of the situation among families of the passengers. China’s foreign minister, Wang Yi, seemed impatient in a phone call reported on the ministry’s website.

"The Malaysian Airlines flight has been missing for close to 40 hours," he was quoted as telling his Malaysian counterpart, Anifah Haji Aman. "The Chinese government is treating this very seriously, and the Chinese people are extremely anxious."

Vietnamese media reported that a yellow object that rescuers thought might be part of the aircraft turned out to be a coral reef.

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